although the regulation of microtubule assembly in cells is not understood, microtubules do appear to assemble at their ends. The ends of both assembling and fully grown ciliary and flagellar microtubules are bound to capping structures. Specific structures are bound to the 2 central and 9 doublet microtubules and, in some organisms, structures attached to different classes of cilia on a single cell or to individual microtubules within a single cilium are morphologically distinguishable. The capping structures may be involved in the initiation or regulation of microtubule assembly because they are attached to what is believed to be the microtubule assembly site. In cells or cilia with more than one type of capping structure, the microtubules to which they are attached differ in the time of assembly or disassembly or in the length of the microtubules. One of the goals of this project is to fully characterize the morphology, assembly, and association of the capping structures with growing and fully assembled microtubules. Biochemical studies of the isolated structrues will lead to an investigation of cytoplasmic microtubules for the presence of similar structures or proteins. These studies will contribute to an understanding of the mechanisms that regulate both the initiation and final length of ciliary and cytoplasmic microtubules in vivo. The function of vertebrate tracheal cilia is to transport mucus over the epithelial surface. The capping structures in these cilia bind each of the microtubules together and to the ciliary crown. By contrast, the capping structures in commonly studied invertebrate cilia do not link the tips of all microtubules. One consequence of this linkage is to limit the total amount of microtubule sliding that can occur, which suggests that the control of sliding in tracheal cilia may be different from that in invertebrates. A second consequence is that the ciliary crown is moved over the tips of the cilia at the site of contact between the cilia and mucus. The second major goal of this project is to determine the manner by which the cap alters the sliding of microtubules in tracheal cilia and how this alteration and the movement of the crown facilitates the transport of mucus over the tracheal epithelium.